Characterization of semiconductor resist material during processing

ABSTRACT

Several methods for evaluating certain changes induced in a film of semiconductor resist material after the coat process and before or at completion of the softbake process, by monitoring thickness of and absorption by a film of resist material.

FIELD OF THE INVENTION

This invention relates to determination of semiconductor resist materialparameters such as percentage of solvent remaining during resistfabrication processes such as softbake.

BACKGROUND OF THE INVENTION

Photoresist and other resist material such as electron beam or ion beamresist is used in semiconductor processing to fabricate masks that allowetching of selected portions of a chip at any level. Resist materialsare usually organic polymers that change state in response toirradiation of small portions of the resist material by a beam ofphotons (visible, ultraviolet, X-ray) or charged particles such aselectrons or ions. In a positive resist material, irradiation produceslocal decomposition of the resist constituents into lower molecularweight polymers so that the etchant attacks these areas more readilythan it attacks constituents in the unirradiated areas. In a negativeresist material, irradiation produces cross-linking among the polymersso that the resulting higher molecular weight constituents resistetching action more successfully than do constituents in theunirradiated areas. This cross-linking produces a swelling of negativeresist material so that optical resolution in a negative resist isusually limited to 1.5 μm or more. However, positive and negative resistmaterials are initially fabricated in a similar manner. The substrate isoften oxidized or unoxidized Si or GaAs, doped or undoped.

SUMMARY OF THE INVENTION

One purpose of this invention is to provide a method for quantitativelymonitoring certain changes induced in a film of resist material as thematerial is processed in the resist coating and/or softbake operations.

Other purposes of the invention, and advantages thereof, will becomeclear by reference to the detailed description and accompanyingdrawings.

To achieve the purposes of the invention, the method in one embodimentmay include the steps of: determining the thickness of the resist filmthrough a measurement of Total Reflectivity or Total Transmissivity ofthe resist film at each of a sequence of one or more predetermined wavelengths λ₁, λ₂, . . . , λ_(r) at a sequence of one or more predeterminedfilm incidence angles θ₁,1, θ₁,2, . . . , θ₁,s (with r+s≧3); anddetermining the absorption of the resist material after the coat processand before completion of the softbake process through a measurement ofTotal Reflectivity or Total Transmissivity of the resist film for apredetermined wavelength λ, a predetermined film incidence angle θ₁ andpredetermined resist film thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic view of typical resist dose D (expressed inmillijoules/cm²) required to adequately expose processed resist material1 μm thick after the resist has been softbaked at temperature T_(soft)for 10-30 minutes, for two positive photoresist materials.

FIG. 2 is a graphic view illustrating rate of dissolution R_(d) (inarbitrary units) of a representative resist versus softbake temperatureT_(soft).

FIG. 3 is a graphic view of weight percentage of solvent remaining aftersoftbake for 30 minutes at temperature T_(soft).

FIG. 4 is a graphic view of changes in resist thickness after softbakeversus T_(soft).

FIG. 5 is a graphic view of the change of refractive index withincreasing dose, for five resist materials.

FIGS. 6, 7 and 8 are graphic views illustrating percentage transmissionfor three American Hoechst resist materials, AZ1370, AZ4110 and AZ5214-Efor resist thicknesses of 1 μm, 1 μm and 1.15 μm, respectively.

DETAILED DESCRIPTION

Photoresist and other resist material is normally applied and used inthree or more phases. First, the resist liquid (or gel) is applied at anapproximate thickness (usually 100 μm or more) to one planar surface ofa substrate, and the substrate is spun about a central axis to throw offa portion of the resist by centrifugal force and reduce the resist filmthickness to a first thickness, as yet unknown. At this point the resistincludes both the sensitizer (solute) and the more volatile solvent orliquid vehicle; the solvent continues to evaporate so that filmthickness varies with time. If the resist material (with or without thesubstrate present) is illuminated by electromagnetic radiation (light)of wavelength λ, for broad ranges of λ the solvent is substantiallytransparent and any absorption in the resist material is duesubstantially wholly to the solute present in the resist material. Thus,if a fixed amount of solute is uniformly distributed in the resistsolvent, the absorption will be independent of resist thickness; and ameasurement of absorption will not vary as solvent (only) is removedfrom the resist material. Here, absorption of the resist film materialis taken to be a=1-e⁻αL, where L is the length of the optical path ofthe light within the resist material and α=α(λ) is the effectiveabsorptivity coefficient (expressed in units of μm⁻¹) for propagation oflight to wavelength λ through the resist material. This definition ofabsorption uses the well-known Beers-Lambert law for absorption oflight, which is a reasonably accurate first approximation for suchabsorption.

Second, the solvent volatilises and leaves the resist solution, throughsoftbake by infrared or microwave irradiation or evaporation at highambient temperature for a relatively short time interval (10-60seconds). The fraction of the resist liquid decreases during softbakefrom 30-60 weight percent before softbake to 10-20 weight percent at thecompletion of softbake. This is sometimes referred to as "curing" theresist.

Third, after the resist solvent weight percent has been reducedsufficiently so that the remaining resist solution forms a mechanicallystable material, a portion of the resist is masked and the remainder isexposed to a light beam at an appropriate wavelength or to a chargedparticle beam (for example, electrons or ions) at an appropriate kineticenergy, and subsequently developed or etched. If too much of the solventremains after softbake and before exposure, a positive resist willbecome underexposed and a negative resist will incompletely polymerize;and both will manifest poor etch resistance. This behavior is discussedby D. J. Elliott in Integrated Circuit Fabrication Technology,McGraw-Hill, 1982, pp. 125-229, which is incorporated herein byreference.

With reference to phase one, a theoretical equation

h=resist thickness after spin=kp² /(ω)^(1/2),

p=percentage of solids in resist,

ω=spin rate (rpm),

k=spin-coater constant,

is known to be inaccurate for many of the spray and roller coatingenvironments used in preparation of resist films and for different spintimes (normally, 10-30 seconds). Thus, it is of interest toindependently determine the resist film thickness after completion ofthe coating process, and possibly at various times during and atcompletion of the softbake process. Further, the percentage of solute,especially of the sensitizer, present in the coated resist film and itsuniformity of distribution throughout the film need to be determined.The subject invention allows accurate measurements of film thickness andfilm absorption at any time after completion of the coating process.

With reference to phase two, when resist solvent is removed by softbake(at temperatures T=60°-140° C.), a very small portion of the sensitizeris also removed or converted by this process, and this portion appearsto increase monotonically with increasing softbake temperature T_(soft).As the fraction converted increases, the total exposure or dose requiredfor optimum contrast or line resolution also rises, as illustrated inFIG. 1 for two typical positive resist families of materials, ShipleyAZ-111 and Shipley AZ-1300. Positive resists that are under-softbakedare more easily attacked by the developer in both exposed and unexposed(masked) areas; although this may indicate increased sensitivity orphotosensitivity, resolution suffers here as image and nonimage regionsare both attacked and etched, and contrast suffers. Negative resistmaterials have a similar problem; under-softbaking leads to incompletepolymerization in exposed areas of the resist, and the exposed andunexposed areas are attacked more easily, with a concomitant fall incontrast. Contrast is defined here as the ratio ##EQU1## where I_(max)and I_(min) are the maximum and minimum radiation intensities,respectively, sensed through the exposed and developed (etched) resist.This characteristic, higher dissolution rate of the resist material asone decreases T_(soft), is illustrated generically in FIG. 2. In apositive resist, dissolution rate R_(d) is determined by T_(soft), bythe time interval Δt_(soft) of softbake, and by the manner (convection,conduction, infrared, microwave, rf, etc.) in which the thermal energyis delivered. In a negative resist, R_(d) depends upon all thesevariables and also upon degree of polymerization of the resist material.

At the other extreme, if the softbake temperature is too high, or if thesoftbake time interval Δt_(soft) is too long (for example, T_(soft)>T_(threshold) ≃120° C.), the solute will become degraded and lose someof its sensitivity. In practice, this often limits softbake temperaturesto a range of 80° C.-120° C. and to time intervals Δt_(soft) =0.2-60minutes. Softback temperature appears to have more of an effect thansoftbake time on thickness of a resist material coated on a substrate.

FIG. 3 graphically illustrates remaining solvent content(weight-percent) after a time interval Δt_(soft) =30 min. at a sequenceof temperatures T_(soft) =60° C.-140° C. for three representative resistmaterials, Shipley AZ-1350J, Hunt Waycoat HPR 204 and Kodak Micro Resist809. Note the precipitous fall in remaining solvent content as T_(soft)increases above 120° C. This accelerated loss of solvent, possibly duein part to conversion of solute molecules, leads to a reduction inresist thickness that can be correlated with T_(soft), for fixedΔt_(soft) and fixed manner of delivery of thermal energy, as illustratedin FIG. 4 for a representative material such as AZ-1350J. In FIG. 4, theresist thickness may change relatively little (by about two percent) inthe range 60° C.≦T_(soft) ≦100° C., but this thickness appears to changesharply above or below this temperature range.

The solvent component of the resist is substantially transparent for thewavelengths of interest (λ=0.3-0.5 μm) for electromagnetic radiationexposure in the third phase. The solute alone contributes to absorptionat such wavelengths, but both solute and solvent may contribute to thereal component of the refractive index n=n_(r) +ιn_(i) (ι² =-1) of theresist solution. Here, the solvent percentage varies over a narrowrange, over which the refractive index of the solute/solvent combinationis substantially constant and thus can be determined ab initio. Therefractive index of a resist material appears to decrease monotonicallywith increasing dose (mJ/cm²), as illustrated in FIG. 5 for five resistmaterials, Shipley AZ-1350B, 1350J, 1370, 4110 and 5214.

Eastman Kodak, in its data sheets on Kodak Micro Positive Resist 809,notes that the refractive index n of resist 809 increases withincreasing T_(softbake) ; for example, n increases by approximately 0.02as T_(softbake) increases from 70° C. to 100° C. If the photoresistmaterial has a well defined refractive index n, standing waves candevelop for certain resist thicknesses h= ##EQU2## (k=0,1,2, . . . )that are illuminated with monochromatic light of wavelength λ; radiationfield intensity nulls or minima will occur at regularly spaced intervalsacross the resist thickness, the result of destructive interference ofthe light waves. This manifests itself as a scalloped pattern thatsharply degrades mask or line width control.

A central idea here is to use changes in particular chemical andphysical characteristics of the resist material as it is processed toindependently monitor or evaluate the quality of the coating process andthe softbake process.

The invention is applicable to positive resist and photoresistmaterials, including Kodak Micro Resist 809, Hunt Waycoat HPR 204 and206, Ohka OFPR-800, MIT IC 528, and Shipley AZ 111S, 111H, 1350J, 1370,1450J and 2400. The invention is also applicable to negative resist andphotoresist materials, including Kodak Micro Resist 747 and 752, HuntWaycoat HNR 80, HNR 120, SC 100 and HR 100, MIT Isopoly MR and HD, OhkaOSR, and KTI 732, 747 and 752. Positive and negative resists usedifferent solvents as well as different photosensitizers. For example,the AZ 1350 and 1370 positive photoresists use 2-ethoxylethyl acetate,and AZ 1512 and 1518 use propylene glycol monoethyl ether acetate assolvent; and the KTI negative photoresists 732, 747 and 752 use xyleneas solvent. Typical recommended doses of positive resist materials(e.g., 80-256 millijoules/cm² for Kodak Micro Resist 809) appear to betwo to five times as large as typical recommended doses for negativeresist materials of similar thicknesses and wavelengths (e.g., 45-55millijoules/cm² for Kodak 747).

Copending U.S. Pat. applications Ser. No. 07/134,638 by R. V. Tan et al.(filed 18 Dec. 1987) and Ser. No. 07/135,119 by D. W. Myers et al.(filed 18 Dec. 1987) disclosed inventions for determining the thicknessand refractive index (at a predetermined wavelength) of a film ofmaterial and for determining the absorption corresponding to propagationof light of a predetermined wavelength π through a film of material ofknown thickness and refractive index, when the film is mounted on asubstrate of known refractive index at that wavelength. The disclosuresof these two applications are incorporated herein by reference.

The Tan et al. application discloses a first method (herein called the"Reflectivity Inversion Method") for determining thickness h of the film(having predetermined refractive index n₂ (λ₁)) by appropriatelyinverting the equation ##EQU3## for each of a sequence of predeterminedwavelengths λ=λ₁, λ₂, . . . , λ_(w) (w≧2), where θ₁ is a predeterminedincidence angle of the light beam, R_(j) is measured Total Reflectivityof the beam intensity from the film at wavelength λ=λ_(j) (j=1, 2, . . ., w), θ₂ and θ₃ are the angles of refraction in the film and in theunderlying substrate, respectively, n₃ (λ) is the substrate refractiveindex, and r₁₂ (λ) and r₂₃ (λ) are the calculated Fresnel amplitudereflection coefficients for reflection within air (or vacuum) and withinthe film material at an air-film interface and at a film-substrateinterface, respectively, for a predetermined beam polarization (p-waveor s-wave).

A second method (herein called the "Reflectivity Versus WavelengthVariance Method) for thickness determination dcisclosed in the Tan etal. application begins with a fixed incidence angle θ₁, a sequence ofpredetermined film thickness numbers {h₁ }^(M) _(i=1) with h_(min) ≦h₁<h₂ <. . . <h_(M) ≦h_(max), and a sequence of predetermined wavelengthsλ₁, λ₂, . . . λ_(w) and (1) generates two computed arrays of numbers##EQU4## (2) forms a sequence {X_(R) (h_(i))}^(M) _(i=1) of variancenumbers ##EQU5## where R'(θ₁ ; λ_(j))=measured film reflectivity atwavelength λ_(j), and (3) chooses a value of film thickness h=h_(k) forwhich X_(R) (h_(k)) is a minimum among the sequence {X_(R) (h_(i))}_(i).

In a third method (herein called the Reflectivity Versus Incidence AngleVariance Method") for thickness determination, the Total Reflectivityvalues R are computed for each of a sequence of predetermined incidenceangles {θ₁,g }^(v) _(g=1) (v≧2) and a fixed wavelength λ₁ ; this yieldsan array of Total Reflectivity values R (θ₁,g ; λ₁ ; h_(i)), where h_(i)as before is drawn from a sequence of predetermined thickness numbers{h_(i) }^(M) _(i=1). Reflectivity measurements are made on thefilm/substrate combination with unknown film thickness at each incidenceangle θ₁,g at the wavelength λ₁ ; these yield a sequence of measurements{r(θ₁,g ; λ₁)}^(v) _(g=1). One then computes the variance ##EQU6## andchooses as the film thickness h the thickness h_(i) corresponding tominimum variance; if two or more distinct values of thickness h yieldsubstantially the same variance X_(R) (h_(i) |θ), other criteria shouldbe invoked to determine which of these values of granular thickness ispreferred.

All three of these methods rely on measurement of Total Reflectivity R,defined here as ##EQU7## where r₁₂ and r₂₃ are the Fresnel amplitudereflection coefficients. These methods, and any other method ofdetermining thickness of a film mounted on a substrate by measurementand use of Total Reflectivity at a sequence of wavelengths or a sequenceof film incidence angles, may be used with the invention disclosedherein.

The Myers et al. application discloses four methods for determining theabsorption a=1-e⁻αL, corresponding to propagation of light of apredetermined wavelength λ along an optical path of length L through asubstantially planar film of material of known refractive index n₂ (λ)(real or complex) and thickness h₂ that is mounted on a substrate havingknown refractive index n₃ (λ). In a first method (herein called the"Simple Reflectivity Method"), n₂ is assumed to be real, and onepropagates the light beam through air (or vacuum) to the air-filminterface at a predetermined incidence angle θ₁ and measures the TotalReflectivity R=I_(reflected) /I_(incident) of the incident beamintensity and determines the absorption a substantially from theexpression ##EQU8## where r₁₂ and r₂₃ are the (real) Fresnel amplitudereflection coefficients for the film-air interface and thefilm-substrate interface, respectively, both within the film material,for the chosen polarization of the incident beam.

In a second method (herein called the "Complex Reflectivity Method"),the (complex) film and substrate refractive indices n₂ =n_(2r) +ιn_(2i)(known) and n₃ =n_(3r) +ιn_(3i) (known) with ι² =-1 are used todetermine each component of the complex refraction angles θ_(a) =θ_(ar)+ιθ_(ai) (a=2,3) by manipulation of the relation r_(ir) sin θ₁ =(n_(ar)+ιn_(ai)) (sin θ_(ar) cos hθ_(ai) +θcos θ_(ar) sin hθ_(ai)) (a=2,3). Thecomplex Fresnel amplitude reflection and transmission coefficientsr_(ij) and t_(ij) (i,j=1,2,3; i≠j) are determined as ratios of solutionsof certain linear equations (boundary conditions) between the electricand magnetic field strength variables E and H. Measurement of TotalReflectivity R_(meas) at the air-film interface and comparison with thecalculated Total Reflectivity ##EQU9## yields a quadratic equation inexp (-αL) whose solution is

    1-a=exp(-αL)=[u±(u.sup.2 -tv).sup.1/2 ]/t

where t, u and v are polynomials on the variables R_(meas), r_(ij) andt_(ij) (i,j=1,2,3; i≠j) computed in the Myers et al. application.

In a third method (the "Simple Transmissivity Method"), the TotalTransmission T_(meas) =I_(transmitted) /I_(incident) (through film andsubstrate) of incident beam intensity is measured, and one determinesthe absorption a=1-e⁻α.sbsp.2^(L).sbsp.2 substantially from theexpressions ##EQU10## L₂ =one-pass optical path length in film=t₂ secθ₂, α₂ =absorption coefficient for film material,

F/2={e²α.sbsp.3^(L).sbsp.3 -2r₃₁ r₃₂ cos β₃ +r₃₁ ² r₃₂ ²e⁻²α.sbsp.3^(L).sbsp.3 }/(1-r₁₂ ²)(1-r₂₃ ²)(1-r₃₁ ²),

β₃ =4πn₃ t₃ /λcos θ₃,

t₃ =substrate thickness,

L₃ =one-pass optical path length in substrate=t₃ secθ₃,

α₃ =absorption coefficient for substrate material,

where the (real) Fresnel amplitude reflection coefficients r_(ij) and β₂and L₂ are computed as in the first method.

In a fourth method (the "Complex Transmissivity Method"), the complexfilm and substrate refractive indices n₂ =n_(2r) +ιn_(2i) (known) and n₃=n_(3r) +ιn_(3i) (known) are used to determine each component of thecomplex refraction angles θ_(a) =θ_(ar) +ιθ_(ai) (a=2,3) and the complexFresnel amplitude reflection and transmission coefficients r_(ij) andt_(ij) (i,j=1,2,3; i≠j) as in the second method. The absorptiona=1-e⁻α.sbsp.2^(L).sbsp.2 is determined substantially from theexpressions

    (1-a).sup.2 =e.sup.-2α.sbsp.2.sup.L.sbsp.2 =[u'±(u'.sup.2 -t'v').sup.1/2 ]/t',

where t', u' and v' are polynomials in the variables T_(meas), r_(ij)and t_(ij) (i,j=1,2,3; i≠j) computed in the Myers et al. application.

The absorption a_(o) =1-e⁻(αL)o of the bare substrate surface is firstmeasured and stored. Here, the single parameter (αL)_(o) measures anyabsorption or related anomalies that occur at the surface of the baresubstrate, based upon substrate refractive index and theoreticalreflectivity of the substrate surface.

The resist material is then deposited on one or a plurality of wafers(20 is a convenient batch size) at estimated thicknesses of 1 μm, andthe wafers are spun. The wafers are then allowed to settle for a periodof time (optional), and coating thickness is measured for each wafer,using measurement of Total Reflectivity such as the ReflectivityInversion Method, the Reflectivity Versus Wavelength Variance Method orthe Reflectivity Versus Incidence Angle Variance Method discussed above,or any other suitable method such as ellipsometry. The wavelength(s) λof irradiation for the thickness determination should be chosen so thatthe refractive index n(λ) is real, or substantially so. For many resistmaterials, the refractive index n=n_(r) +ιn_(i) (ι² =-1) satisfies n_(i)<0.01 for all λ>0.5 μm so that this constraint is easily met.

FIGS. 6, 7 and 8 illustrate the percentage transmission of threerepresentative Hoechst positive resist materials, AZ 1370, AZ 4110 andAZ 5214-E as a function of irradiation wavelength λ for thicknesses of 1μm, 1 μm and 1.15 μm, respectively. From Eqs. (16)-(21) of the Myers etal. Application, the propagation vector q appears in the form ##EQU11##where E is the electric field strength vector and ω=2πc/λ is the angularvelocity corresponding to the irradiation wavelength. The intensitytransmission factor τ corresponding to propagation of a light wavethrough a thickness h of such material then becomes Y exp(-2ωhn_(i) /c)so that the imaginary component of the refractive index n_(i) becomes

    n.sub.i =(λ/4πh) 1n(1/π).

Table I presents the computed values of n_(i) for AZ 1370, AZ 4110 andAZ 5214-E for some representative wavelengths λ=0.365 μm, 0.405 μm,0.442 μm and 0.5 μm, for unexposed and completely exposed resistmaterials. For each resist material there exists a "window", in thewavelength range within which n_(i) (unexposed) differs substantiallyfrom n_(i) (completely exposed); this window is given approximately by0.3 μm <λ<0.48 μm for the three resist materials shown in FIGS. 6, 7 and8.

                  TABLE I.                                                        ______________________________________                                        Imaginary Component of Refractive Index (n.sub.i).                                   λ =                                                                           λ = λ =                                                                             λ =                                         0.365 μm                                                                          0.405 μm                                                                              0.442 μm                                                                            0.05 μm                                  ______________________________________                                        AZ 1370                                                                       n.sub.i (unexp.)                                                                       0.0467   0.0410     0.0244 0.0060                                    n.sub.i (exposed)                                                                      0.0148   0.0093     0.0074 0.0060                                    AZ 4110                                                                       n.sub.i (unexp.)                                                                       0.0226   0.0223     0.0131 0.0042                                    n.sub.i (exposed)                                                                      0.0058   0.0049     0.0049 0.0042                                    AZ 5214-E                                                                     n.sub.i (unexp.)                                                                       0.0142   0.0157     0.0019   0                                       n.sub.i (exposed)                                                                      0.0130     0          0      0                                       ______________________________________                                    

The wafers are then measured for absorption a=1-e⁻αL (L=single passoptical path length, α=absorptivity coefficient), using a measurement ofTotal Reflectivity or of Total Transmissivity for the substrate/resistcoat combination. This method may be drawn from the Simple ReflectivityMethod, the Complex Reflectivity Method, the Simple TransmissivityMethod, the Complex Transmissivity Method, or any other suitable methodthat works with all orders of optical interferences, to determine a orα. As noted above, the solvent is substantially transparent to radiationof wavelengths λ=0.3-0.5 μm so that the absorptivity coefficient αshould remain substantially the same throughout the process of (partial)removal of the solvent. The thickness h of, and optical path length Lwithin, the resist material are related approximately by the equation

    L=h sec θ.sub.2 =h/[1-sin .sup.2 θ.sub.1 /n.sub.2r.sup.2 ].sup.1/2,

where θ₁ is the incidence angle, θ₂ is the corresponding refractionangle in the resist material and n_(2r) is the real component of therefractive index of the resist material.

The fact that the resist solvent is substantially transparent at thewavelengths of interest allows one to decouple an evaluation of the coatprocess, using determinations of thickness h and absorptivitycoefficient α after coating and before softbake, from an evaluation ofthe softbake process, using post-softbake values for thickness h. Forthe coat process, one useful parameter is

    Δ=αL (post-coat, pre-softbake)-(αL).sub.o (pre-coat).

Forming the difference Δ cancels out any effect on absorptivity due toanomalies at the substrate surface. Ideally, Δ should be the same foreach wafer, indicating that the solute molecules are uniformlydistributed throughout the batch of wafers being tested. The parameter Δrepresents "true" absorptivity of the resist material itself; Δ for eachwafer should lie within a narrow range, say ± one percent of the targetvalue Δ_(t) for that resist material. Alternatively, Δ_(t) may be takenas the arithmetic mean of all the values of Δ in this batch.

For the softbake process a useful parameter is

    ρ=h(after softbake)/Δ

This provides a measure of absorption in the resist film due to thesolute molecules, which are assumed to be substantially the onlymolecules that contribute to absorption. One might, for example, set atarget ρ_(t) for ρ after softbake and compare each ρ with ρ_(t) forconformity. Alternatively, one might set ρ_(t) equal to the arithmeticmean of the ρ values for all of the wafers and compare each ρ with themeans value for the process.

The most important feature here is the fact that the coat and softbakeprocesses can now be monitored separately and independently forcompliance with predetermined standards for each of these processes.This approach resolves two problems perceived by many workers in thisfield: (1) After coat and before softbake stabilizes the resist film,the thickness h continues to vary with time as evaporation proceeds; onesolution is to use the absorptivity coefficient α, which is assumed toarise from the presence of only the solute molecules in the resist ofthe irradiation wavelength chosen; (2) During the softbake process, onehas two parameters (Δ and h) to work with, with differing units; onesolution here is to form the product or the ratio of these two monitorparameters.

Although the preferred embodiments of the invention have been shown anddescribed herein, modification and variation can be made withoutdeparting from the scope of the invention.

We claim:
 1. A method for evaluating the quality of a semiconductor resist material produced by a softbake process, that contains a solute and a solvent and that is deposited as a substantially planar film on a substrate, the method comprising the steps of:determining the absorption a=1-e⁻α.sbsp.o^(L).sbsp.o (L_(o) =single pass optical path length in the film of interest, if any) and absorption coefficient (αL)_(o) of the substrate surface before the coat process begins, through a measurement of Total Reflectivity of the substrate surface for a predetermined wavelength λ, predetermined incidence angle θ₁ and known index of refraction of the substrate; depositing a resist film on the substrate at an estimated predetermined thickness; determining the thickness of the resist film through a measurement of Total Reflectivity of the resist film at each of a sequence of one or more predetermined wavelengths λ₁, λ₂, . . . , λ_(r) at a sequence of one or more predetermined film incidence angles θ₁,1, θ₁,2, . . . , θ₁,s (with r+s≧3); determining the absorption of a=1-e⁻αL of the resist film after the coat process and before completion of the softbake process through a measurement of Total Reflectivity of the resist film for a predetermined wavelength λ, predetermined film incidence angle θ₁, and predetermined film thickness h; forming the quantity Δ=αL-(αL)_(o) for each of the sequenmce of combinations of substrate and resist film deposited thereon; comparing the value of Δ for each such combination with a predetermined target value Δ_(t) ; performing the softbake process for each such combination; forming the quantity ρ=h/Δ for each of the sequence of combinations of substrate and resist film deposited thereon; and comparing the value of ρ for each such combination with a predetermined target value ρ_(t).
 2. The method according to claim 1, wherein said step of determining said thickness of said resist film is carried out using a method drawn from the class consisting of the Reflectivity Inversion Method, the Reflectivity Versus Wavelength Variance Method, and the Reflectivity Versus Incidence Angle Variance Method.
 3. The method according to claim 1, wherein said step of determining said absorption of said resist film is carried out using a method drawn from the class consisting of the Simple Reflectivity Method and the Complex Reflectivity Method.
 4. The method according to claim 1, wherein said target value Δ_(t) is the arithmetic mean of the values of said parameter Δ determined for said sequence of combinations of substrate and resist film.
 5. The method according to claim 1, wherein said target value ρ_(t) is the arithmetic mean of the values of said parameter ρ determined for said sequence of combinations of substrate and resist film.
 6. A method for evaluating the quality of a semiconductor resist material, produced by a softbake process, that contains a solute and a solvent and that is deposited as a substantially planar film on a substrate, the method comprising the steps of:determining the absorption a=1-e⁻α.sbsp.o^(L).sbsp.o (L_(o) =single pass optical path length in the film of interest, if any) and absorption coefficient (αL)_(o) of the substrate surface before the coat process begins, through a measurement of Total Transmissivity of the substrate surface for a predetermined wavelength λ, predetermined incidence angle θ₁ and known index of refraction of the substrate; depositing a resist film on the substrate at an estimated predetermined thickness; determining the thickness of the resist film through a measurement of Total Reflectivity of the resist film at each of a sequence of one or more predetermined wavelengths λ₁, λ₂, . . . , λ_(r) at a sequence of one or more predetermined film incidence angles θ₁,1, θ₁,2, . . . θ₁,s (with r+s≧3); determining the absorption a=1-e⁻αL of the resist film after the coat process and before completion of the softbake process through a measurement of Total Transmissivity of the resist film for a predetermined wavelength λ, predetermined film incidence angle θ₁, and predetermined film thickness h; forming the quantity Δ=αL-(αL)_(o) for each of the sequence of combinations of substrate and resist film deposited thereon; comparing the value of Δ for each such combination with a predetermined target value Δ_(t) ; performing the softbake process for each such combination; forming the quantity ρ=h/Δ for each of the sequence of combinations of substrate and resist film deposited thereon; and comparing the value of ρ for each such combination with a predetermined target value ρ_(t).
 7. The method according to claim 6, wherein said step of determining said thickness of said resist film is carried out using a method drawn from the class consisting of the Reflectivity Inversion Method, the Reflectivity Versus Wavelength Variance Method, and the Reflectivity Versus Incidence Angle Variance Method.
 8. The method according to claim 6, wherein said step of determining said absorption of said resist film is carried out using a method drawn from the class consisting of the Simple Transmissivity Method and the Complex Transmissivity Method.
 9. The method according to claim 6, wherein said target value Δ_(t) is the arithmetic mean of the values of said parameter Δ determined for said sequence of combinations of substrate and resist film.
 10. The method according to claim 6, wherein said target value ρ_(t) is the arithmetic mean of the values of said parameter ρ determined for said sequence of combinations of substrate and resist film. 